I'm fine with wearing suits on sales calls in the right contexts, but it's more about wearing what is appropriate. At YC (or really any tech event on the west coast), a suit isn't appropriate -- at most, a button-down shirt w/o tie and MAYBE a jacket in the evenings.
It's kind of obvious when the guy on the other side of the table is wearing shorts and sandals that perhaps a suit and tie is inappropriate...
I can't speak for the US west coast, but it seems to me a good start is to actually know how to wear a suit. It absolutely has to fit correctly and be the right size. Unbutton your jacket when you sit. Tie your tie correctly and tighten and straighten it, top button of your shirt buttoned - OR loose it completely, opening the top button - there is no middle ground.
It's kind of obvious when the guy on the other side of the table is wearing shorts and sandals that perhaps a suit and tie is inappropriate...